diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -208,11 +208,6 @@
  *   The filename of the requested item or NULL if the item is not found.
  */
 function drupal_get_filename($type, $name, $filename = NULL) {
-  // Return NULL right away if $type or $name is empty.
-  if (empty($type) || empty($name)) {
-    return NULL;
-  }
-
   // The location of files will not change during the request, so do not use
   // drupal_static().
   static $files = array();
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -157,17 +157,23 @@ public function installDefaultConfig($type, $name) {
    * {@inheritdoc}
    */
   public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = []) {
+    $profile = $this->drupalGetProfile();
     if (!$storage) {
       // Search the install profile's optional configuration too.
       $storage = new ExtensionInstallStorage($this->getActiveStorages(StorageInterface::DEFAULT_COLLECTION), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, StorageInterface::DEFAULT_COLLECTION, TRUE);
       // The extension install storage ensures that overrides are used.
       $profile_storage = NULL;
     }
-    else {
+    elseif (isset($profile)) {
       // Creates a profile storage to search for overrides.
-      $profile_install_path = $this->drupalGetPath('module', $this->drupalGetProfile()) . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
+      $profile_install_path = $this->drupalGetPath('module', $profile) . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
       $profile_storage = new FileStorage($profile_install_path, StorageInterface::DEFAULT_COLLECTION);
     }
+    else {
+      // Profile has not been set yet. For example during the first steps of the
+      // installer or during unit tests.
+      $profile_storage = NULL;
+    }
 
     $collection_info = $this->configManager->getConfigCollectionInfo();
     $enabled_extensions = $this->getEnabledExtensions();
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
+++ b/core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
@@ -98,8 +98,11 @@ protected function getAllFolders() {
         // The install profile can override module default configuration. We do
         // this by replacing the config file path from the module/theme with the
         // install profile version if there are any duplicates.
-        $profile_folders = $this->getComponentNames('profile', array(drupal_get_profile()));
-        $this->folders = $profile_folders + $this->folders;
+        $profile = drupal_get_profile();
+        if (isset($profile)) {
+          $profile_folders = $this->getComponentNames('profile', array($profile));
+          $this->folders = $profile_folders + $this->folders;
+        }
       }
     }
     return $this->folders;
only in patch2:
unchanged:
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -82,7 +82,7 @@ function _ckeditor_theme_css($theme = NULL) {
   if (!isset($theme)) {
     $theme = \Drupal::config('system.theme')->get('default');
   }
-  if ($theme_path = drupal_get_path('theme', $theme)) {
+  if (isset($theme) && $theme_path = drupal_get_path('theme', $theme)) {
     $info = system_get_info('theme', $theme);
     if (isset($info['ckeditor_stylesheets'])) {
       $css = $info['ckeditor_stylesheets'];
only in patch2:
unchanged:
--- a/core/modules/quickedit/quickedit.module
+++ b/core/modules/quickedit/quickedit.module
@@ -77,7 +77,7 @@ function quickedit_library_info_alter(&$libraries, $extension) {
 
     // First let the base theme modify the library, then the actual theme.
     $alter_library = function(&$library, $theme) use (&$alter_library) {
-      if ($theme_path = drupal_get_path('theme', $theme)) {
+      if (isset($theme) && $theme_path = drupal_get_path('theme', $theme)) {
         $info = system_get_info('theme', $theme);
         // Recurse to process base theme(s) first.
         if (isset($info['base theme'])) {